1. Field of the Invention
The present invention relates to a driving device capable of improving display quality in a liquid crystal display apparatus such as a liquid crystal panel of simple matrix type.
2. Description of The Related Art
FIG. 39 shows a schematic electrical configuration for driving a simple matrix type liquid crystal panel 101 of one prior art. A plurality of segment electrodes of the liquid crystal panel 101 are driven in parallel by a segment side drive circuit 102, and a plurality of common electrodes are driven by a common side drive circuit 103 while being selected sequentially. Power voltages supplied from a power supply circuit 104 to the segment side drive circuit 102 and to the common side drive circuit 103 are six voltages V0, V1, V2, V3, V4 and V5, having a relation of V0&gt;V1&gt;V2&gt;V3&gt;V4&gt;V5. The segment side drive circuit 102 is supplied with four voltages V0, V2, V3 and V5, and the common side drive circuit 103 is supplied with four voltages V0, V1, V4 and V5.
Display data which represents an image to be displayed on the liquid crystal panel 101 is given to the segment side drive circuit 102 as serial data by a controller 105. Data latch clock for latching the display data in synchronization with the display data, horizontal synchronization signal and AC-converting signal are also supplied to the segment side drive circuit 102 from the controller 105. The controller 105 supplies horizontal synchronization signal, vertical synchronization signal and AC-converting signal to the common side drive circuit 103. The common side drive circuit 103 selects a common electrode which should display first in response to a vertical synchronization signal, and thereafter scans in the vertical direction by changing the common electrode to be selected successively while synchronizing with the horizontal synchronization signal.
FIG. 40 shows internal configuration of the segment side drive circuit 102 shown in FIG. 39. The display data supplied from the controller 105 as serial data is converted to parallel data by a shift register 121, latched by the data latch 122 according to a data latch clock, and latched in a line latch 123 at every horizontal scanning period according to the horizontal synchronization signal (LP). Output of the line latch 123 is sent to a liquid crystal drive output circuit 126 via a level shifter 124, together with the AC-converting signal which is sent thereto via a level shifter 125. The level shifters 124, 125 are provided because the operating voltage of the liquid crystal drive output circuit 126 is different from operating voltage Vcc of the shift register 121, the data latch 122 and the line latch 123.
FIG. 41 shows voltage waveforms of various portions and voltage waveform applied to a liquid crystal cell of the liquid crystal panel 101 of the prior art shown in FIG. 39. Although FIG. 41 shows a case with seven scan electrodes for the convenience of description, the actual number of scan electrodes is larger than this. The display data stored in the line latch 123 of the segment side drive circuit 102 is given to the liquid crystal drive output circuit 126 via the level shifter 124. The liquid crystal drive output circuit 126 selects one voltage from among liquid crystal drive voltages V0, V2, V3 and V5 of four levels which are input, on the basis of the display data, and applies the voltage to the segment electrode. The outputs of a segment side drive circuit 102 for one scan electrode are applied to the segment electrodes in parallel. On the other hand, the common side drive circuit 103 supplies liquid crystal drive voltages V0 and V5 from among the four liquid crystal drive power voltages V0, V1, V4 and V5 to a selected common electrode, and supplies liquid crystal drive voltages V1 and V4 to non-selected common electrodes.
The liquid crystal panel 101 comprises common electrodes and segment electrodes which have non-zero resistance, while the liquid crystal layer interposed between the electrodes acts as a dielectric substance and has a non-zero capacitance. Consequently, electrical resistance of each electrode wire and a capacitor formed by a display dot where the liquid crystal works as a dielectric form a low-pass filter. Due to the low-pass filter, voltage drop and rounding of waveform become more significant as the distance from the segment side drive circuit 102 increases. Accordingly a difference in voltage drop and rounding of waveform is caused between a pixel on a scan electrode near to the segment side drive circuit 102 and a pixel on a scan electrode far therefrom, thereby causing a difference in the effective voltage applied to the liquid crystal cell and resulting in a difference in the display density. This difference in the display density causes an upper portion and a lower portion of the liquid crystal display surface to appear having different display densities.
There is a trend to increase panel sizes of liquid crystal display apparatuses are for such needs as replacing CRT monitors of personal computers. Also the standard display for the so-called PC-AT compatible computer is in the trend of increasing the number of display dots as the display standard evolves from VGA to SVGA, and from XGA to SXGA, causing the pixel pitch to decrease. Increasing display screen size causes the pixel and scan electrodes to become longer. Further, trend toward higher pixel resolution causes the widths of the pixel and scan electrodes to decrease. As a result, electrical resistances of the pixel and the scan electrodes increase, thereby causing the difference in the display density to increase further.
As a solution to these problems, for example, such prior art may be applied as proposed in the Japanese Unexamined Patent Publication JP-A 62-43624 (1987). In this prior art, a liquid crystal drive voltage which changes in a saw-tooth form as shown in FIG. 42 is used, thereby to change the voltage waveforms of various portions as shown in FIG. 43. In the case that a high drive voltage is applied at every scanning period, the difference in the density of display between the upper portion and the lower portion of the liquid crystal panel when the segment side drive circuit is installed in the upper portion of the liquid crystal panel can be reduced.
Also for the purpose of driving a simple matrix liquid crystal panel, the present applicant proposed a method of driving the segment side drive circuit with a low voltage, for example to enable it to drive with a single power supply of 5V. Operation with this driving method is shown in FIG. 44. The segment side drive circuit selects and outputs one of two voltages, VSH and VSL, according to a combination of the AC-converting signal and the display data, and determines whether to turn on or off the display. The common side drive circuit selects and outputs one of three voltages VCH, VCM and VCL according to the combination of the AC-converting signal and selection or non-selection.
Comparison of the voltage applied to each liquid crystal cell of the liquid crystal panel between FIG. 41 and FIG. 44 shows that the voltages in both driving methods are identical, provided that the following equations hold. This method of driving will be hereinafter called 5V driving method.
V0-V5=VCH-VSL PA0 V0-V4=VCH-VSM PA0 V0-V3=VCH-VSH PA0 (V4-V4, V1-V1)=(VCH=VSM)=0 PA0 (V4-V5, V1-V2)=VCM-VSL PA0 (V4-V3, V1-V0)=VCM-VSH PA0 V5-V2=VCL-VSL PA0 V5-V1=VCL-VSH PA0 V5-V0=VCL-VSH
With this 5V driving method, too, there arises differences in the density between pixels in the upper portion and lower portion of the liquid crystal panel, as in the prior art described above. The problem of difference in the density can be solved by applying prior art disclosed in JP-A 62-43624.
Disclosed in the Japanese Unexamined Patent Publication JP-A 5-265402 (1993) is prior art of reducing unevenness in brightness of display which is dependent on the display pattern when driving a simple matrix liquid crystal panel. In this prior art, when driving a simple matrix liquid crystal panel, correction periods are provided for all outputs of a column side drive device which corresponds to the segment side at every scanning period of one line, and a correction voltage of an intermediate level between ON display voltage level and OFF display voltage level is output, instead of the display voltage which is output from the column side drive device. According to this prior art, although the unevenness in brightness which depends on the display pattern is reduced, the problem of difference in density between the upper portion and the lower portion of the liquid crystal panel cannot be solved.
In the common side drive circuit, similar to the segment side drive circuit, the drive voltage changes significantly as the distance from the drive circuit increases. Consequently, rounding of the waveform of the drive voltage becomes more significant, resulting in a difference in the density of display between the left side and the right side of the liquid crystal panel. Also since rounding of the waveform of the drive voltage becomes more significant, difference in the effective voltage increases depending on the display pattern. As the difference in the effective voltage increases, shadowing which represents the unevenness in the brightness dependent on the display pattern appears markedly.
Application of the prior art disclosed in JP-A 62-43624 for the elimination of difference in display density due to the distance from the segment side drive device leads to changes in greater voltage range as the distance increases. As a result, rounding of the waveform of the drive voltage becomes more significant and the difference in the effective voltage increases depending on the display pattern, thereby causing shadowing representing the unevenness in the brightness which depends on the display pattern to appear markedly, leading to degradation in the display quality and other problems.
With the prior art disclosed in the JP-A 5-265402, although the unevenness in brightness which depends on the display pattern can be reduced, the problem of difference in display density between the upper portion and the lower portion of the liquid crystal panel, for example, due to the difference in the distance from the drive circuit cannot be solved, resulting in unevenness in density depending on the display area of the display panel which degrades the display quality. Particularly, since a correction period is always provided for every scanning period, frequency of changes in the waveform increases thus leading to increasing effect of rounding of the waveform caused by the increased electrical resistance and increased capacitance due to the increase in the distance, thereby making the unevenness in brightness likely to occur.
In the common side drive circuit, similar to the case of the segment side drive circuit, variation in the drive voltage increases as the distance from the drive circuit increases. Consequently, there has been such a problem as rounding of the drive voltage waveform becomes more significant resulting in difference in the display density between the left side and right side of the liquid crystal panel. Also since rounding of the waveform of the drive voltage becomes more significant, difference in the effective voltage increases depending on the display pattern. As the difference in the effective voltage increases, shadowing representing the unevenness in the brightness which depends on the display pattern appears markedly, resulting in degraded display quality and other problems.